190 Scientific Intelligence. — Natural Philosophy. 



9. Cuvier's explanatUm of accideiital Colours. — M. G. Cii- 

 vier thinks, that the production of all the accidental colours may 

 be explained by this very simple fact, that the retina which has 

 just been subjected to the impression of a colour, becomes, from 

 this very circumstance, incapable of immediately receiving the 

 impression of a fainter colour of the same kind. A very simple 

 experiment, and one which every body has made, without reflect- 

 ing upon it, confirms this truth. When toward evening, one 

 looks to a window, he sees the wood surrounding the panes of a 

 dark colour, while the latter are still light. If, after looking 

 steadily for some time at the window, he turns toward the oppo- 

 site side of the room, which is darker, he sees there an image of 

 the window. This phenomenon can only be explained by ad- 

 mitting, that the part of the retina on which the image of the 

 window is painted, becomes, in consequence of the vivacity of 

 the colours of which the image was formed, incapable of receiv- 

 ing any impression on the part of the dark points of the oppo- 

 site side of the room ; whence results the image seen on the 

 wall. The phenomenon which the retina presents in this case 

 exists more or less with respect to all our senses ; each of which, 

 after being submitted to a rather vivid impression, becomes, 

 from that very circumstance, incapable of experiencing a weaker 

 impression of the same nature. It is enough to eat a bit of su- 

 gar immediately before taking one's coffee, to find that the coffee 

 is not sufficiently sweet. What takes place in this case, with 

 reference to the sense of taste, is analogous to what was observed 

 with respect to the sense of sight in the case of the window. 

 The application of this to the phenomenon of accidental colours 

 is easy. If, for example, an ace of diamonds be fixed on a card, 

 one can only look at it for a very short time, without letting his 

 eyes vacillate to either side. From this moment, the eye, ha- 

 ving become insensible to the red rays, will only see in the white 

 of the card, the green of the band of that colour which sur- 

 rounds the red. What proves the accuracy of this explanation 

 is, that if, after looking at the red ace, one directs his eye to a 

 distant part of the card, he sees a figure of the same form, and 

 of a green colour, the perception of which is owing to the cause 

 already pointed out. 



10. Motiotis of the Magnetic Equator. — M. Morellet ad- 



